Method of forming oxide film, method of manufacturing semiconductor device, and apparatus configured to form oxide film

ABSTRACT

A method of forming an oxide film is provided. The method may include: supplying mist of a solution including a material of the oxide film dissolved therein to a surface of a substrate together with a carrier gas having an oxygen concentration equal to or less than 21 vol % so as to epitaxially grow the oxide film on the surface of the substrate; and bringing the oxide film into contact with a fluid comprising oxygen atoms after the epitaxial growth of the oxide film.

TECHNICAL FIELD

The technology herein disclosed relates to a method of forming an oxidefilm, a method of manufacturing a semiconductor device, and a filingforming apparatus configured to form an oxide film.

BACKGROUND

Japanese Patent Application Publication No. 2015-070248 describes amethod of forming an oxide film. In this film forming method, mist of asolution including a material of the oxide film dissolved therein issupplied to a surface of a substrate. At this occasion, a carrier gas issupplied together with the mist to the surface of the substrate so as tocarry the mist. The mist adheres to the surface of the substrate, andthen the oxide film is epitaxially grown on the surface of thesubstrate.

SUMMARY

In a film forming method using mist, gas having a low oxygen content(e.g., inert gas such as argon or nitrogen) is usually used as a carriergas. This is because if a carrier gas contains a large amount of oxygen,this would cause oxidization of the mist and generation of dust whichwill adhere to a surface of an oxide film. However, if a gas having alow oxygen content is used as the carrier gas, oxygen vacancies (defectsin which oxygen-atom sites are vacant for lack of oxygen atoms) areprone to be generated in a crystal of the oxide film to be epitaxiallygrown. As a result of this, a crystallinity of the oxide film isdeteriorated. Therefore, the disclosure herein proposes a technologyconfigured to form an oxide film having few oxygen vacancies.

The method disclosed herein relates to a method of forming an oxidefilm. The method may comprise supplying mist of a solution including amaterial of the oxide film dissolved therein to a surface of a substratetogether with a carrier gas having an oxygen concentration equal to orless than 21 vol % so as to epitaxially grow the oxide film on thesurface of the substrate, and bringing the oxide film into contact witha fluid comprising oxygen atoms after the epitaxial growth of the oxidefilm.

The above-described fluid includes gas, mist, or the like.

In this film forming method, the gas having an oxygen concentrationequal to or less than 21 vol % is used as the carrier gas. Having theoxygen concentration equal to or less than 21 vol % means that theoxygen concentration of the gas is lower than that in an atmosphere. Thecarrier gas containing less oxygen can prevent the mist from beingoxidized in the epitaxial growth of the oxide film. This also suppressesdust from being generated and adhering to the surface of the oxide film.Since the carrier gas contains few oxygen vacancies are formed in theoxide film to be epitaxially grown. After the oxide film has beenepitaxially grown, the bringing of the oxide film into contact with thefluid containing oxygen atoms is performed. In this step, the oxygenatoms are supplied from the fluid to the oxide film, and the oxygenvacancies in the oxide film are filled with the oxygen atoms. The oxygenvacancies thereby decrease, and the crystallinity of the oxide film isimproved. As such, this manufacturing method is configured to form anoxide film having few oxygen vacancies suitably.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a configuration of a film forming apparatusin a first embodiment;

FIG. 2 is a diagram showing a configuration of a film forming apparatusin a second embodiment; and

FIG. 3 is a diagram showing a configuration of a film forming apparatusin a third embodiment.

DETAILED DESCRIPTION First Embodiment

A film forming apparatus 10 shown in FIG. 1 is an apparatus configuredto epitaxially grow a gallium oxide film on a surface of a substrate 12.The gallium oxide film is a semiconductor film. The film formingapparatus 10 is used for manufacturing a semiconductor device thatincludes the gallium oxide film. The film forming apparatus 10 comprisesa furnace 20 and a reservoir 40.

The reservoir 40 is an enclosed container. The reservoir 40 stores asolution 42 including water (H₂O) and a raw material of the galliumoxide film dissolved in the water. There is a space 44 between a surface42 a of the solution 42 and an upper surface of the reservoir 40. Anultrasonic transducer 48 is arranged at a bottom surface of thereservoir 40. The ultrasonic transducer 48 is configured to applyultrasound to the solution 42 stored in the reservoir 40. When theultrasound is applied to the solution 42, the surface 42 a of thesolution 42 vibrates, by which mist of the solution 42 (hereinaftertermed solution mist 46) is generated into the space 44 above thesolution 42. The upper surface of the reservoir 40 is connected to anupstream end of a mist supply path 30. An outer peripheral wall of thereservoir 40 is connected to a downstream end of a carrier gas supplypath 50. An upstream end of the carrier gas supply path 50 is connectedto a carrier gas supply source (not shown). The carrier gas supply path50 includes a valve 50 a. When the valve 50 a is opened, a first carriergas 52 is introduced from the carrier gas supply source into the space44 in the reservoir 40 via the carrier gas supply path 50. The firstcarrier gas 52 is an inert gas. The first carrier gas 52 has an oxygen(O₂) concentration equal to or less than 21 vol %. More specifically,the first carrier gas 52 contains no oxygen. The first carrier gas 52introduced from the carrier gas supply path 50 into the space 44 flowsfrom the space 44 to the mist supply path 30. At this time, the solutionmist 46 in the space 44 flows to the mist supply path 30 together withthe first carrier gas 52.

A downstream end of a gas supply path 32 is connected to a point in themist supply path 30. An upstream end of the gas supply path 32 isconnected to a carrier gas supply path 34 and an oxygen gas supply path36. An upstream end of the carrier gas supply path 34 is connected to acarrier gas supply source (not shown). The carrier gas supply path 34includes a valve 34 a. When the valve 34 a is opened, a second carriergas 35 is introduced from the carrier gas supply source into the mistsupply path 30 via the carrier gas supply path 34 and the gas supplypath 32. The second carrier gas 35 is an inert gas. The second carriergas 35 has an oxygen concentration equal to or less than 21 vol %. Morespecifically, the second carrier gas 35 contains no oxygen. An upstreamend of the oxygen gas supply path 36 is connected to an oxygen gassupply source (not shown). The oxygen gas supply path 36 includes avalve 36 a. When the valve 36 a is opened, oxygen gas 37 (i.e., O₂) isintroduced from the oxygen gas supply source into the mist supply path30 via the oxygen gas supply path 36 and the gas supply path 32. Apartial pressure of oxygen (O₂) in the oxygen gas 37 is higher than apartial pressure of oxygen (O₂) in an atmosphere.

The furnace 20 includes an intake portion 22, and a channel 24 incommunication with the intake portion 22. The intake portion 22 has ahigher height and the channel 24 has a lower height. The intake portion22 is connected to a downstream end of the mist supply path 30. Anejection pipe 28 is connected to an end of the channel 24. A substratestage 26 is disposed at a lower surface of the channel 24. The substratestage 26 is configured to have the substrate 12 mounted thereon. Aheater 27 is disposed within the substrate stage 26 (i.e., within anouter wall of the furnace 20). The heater 27 is configured to heat thesubstrate 12.

Next, a film forming method using the film forming apparatus 10 will bedescribed. Here, a sapphire substrate is used as the substrate 12, andan α-gallium oxide (Ga₂O₃) semiconductor film is grown on the substrate12. An aqueous solution containing water having a gallium compound(e.g., gallium acetylacetonate, gallium chloride) dissolved therein isused as the solution 42. Further, a tin(II) compound which is a dopantmaterial has been dissolved in the solution 42 to add tin as dopants tothe gallium oxide. Argon (Ar) is used as the first carrier gas 52 andthe second carrier gas 35.

Firstly, the substrate 12 is set on the substrate stage 26. After thesubstrate 12 has been set, a film forming process and an oxygenannealing process are performed.

Firstly, the film forming process is performed. In the film formingprocess, firstly the substrate 12 is heated with the heater 27. Here, atemperature of the substrate 12 is controlled to be 350 to 500° C. Whenthe temperature of the substrate 12 has stabilized, the ultrasonictransducer 48 is activated to generate the solution mist 46 into thespace 44 in the reservoir 40. In doing this, the valve 50 a is opened tointroduce the first carrier gas 52 from the carrier gas supply path 50into the reservoir 40. The solution mist 46 then flows into the mistsupply path 30 together with the first carrier gas 52. Further, thevalve 34 a is opened to introduce the second carrier gas 35 from the gassupply path 32 into the mist supply path 30. As a result of this, thesolution mist 46 is diluted within the mist supply path 30. The solutionmist 46 flows into the furnace 20 together with the first carrier gas 52and the second carrier gas 35. The solution mist 46 flows from theintake portion 22 to the channel 24 together with the first carrier gas52 and the second carrier gas 35, and is ejected to the ejection pipe28. While the solution mist 46 is flowing within the channel 24, a partof the solution mist 46 adheres to the surface of the substrate 12.Since the substrate 12 has been heated with the heater 27, a chemicalreaction of the solution mist 46 (i.e., the solution 42) takes place onthe substrate 12. As a result of this. α-gallium oxide is generated onthe substrate 12. The solution mist 46 is continuously supplied to thesurface of the substrate 12, and accordingly a gallium oxide film(semiconductor film) is grown on the surface of the substrate 12. Asingle-crystal gallium oxide film is epitaxially grown on the surface ofthe substrate 12. The gases supplied to the furnace 20 (i.e., the firstcarrier gas 52 and the second carrier gas 35) have a low oxygen content,and thus the solution mist 46 is less likely to be oxidized. Morespecifically, the gallium oxide compound and the dopant material bothcontained in the solution mist 46 is more difficult to oxidize. Thissuppresses dust from adhering to the surface of the growing galliumoxide film. Meanwhile, since the gas supplied to the furnace 20 (i.e.,the first carrier gas 52 and the second carrier gas 35) has a low oxygencontent, many oxygen vacancies are formed in the gallium oxide filmwhile the gallium oxide film is grown. Moreover, since the solution 42contains the dopant material, the dopants (tin) are captured into thegallium oxide film. The gallium oxide film of an n-type is thus formed.After the gallium oxide film has been formed, the ultrasonic transducer48 is stopped and the valves 34 a, 50 a are closed to stop the supply ofthe solution mist 46 to the substrate 12.

Next, the oxygen annealing process is performed. In the oxygen annealingprocess, firstly the substrate 12 is heated with the heater 27. Here,the temperature of the substrate 12 and the gallium oxide film iscontrolled to be 550° C. or approximately 550° C. In other words, in theoxygen annealing process, the substrate 12 and the gallium oxide filmare heated at a temperature higher than the temperature of the substrate12 in the film forming process. When the temperature of the substrate 12has stabilized, the valve 36 a is opened. The oxygen gas 37 then flowsfrom the oxygen gas supply path 36 into the furnace 20 via the gassupply path 32 and the mist supply path 30. Here, the oxygen gas 37 issupplied at a flow rate of 0.5 L/min. The oxygen gas 37 passes throughthe intake portion 22 and the channel 24, and is ejected to the ejectionpipe 28. Since the oxygen gas 37 flows through the channel 24, thegallium oxide film on the substrate 12 is brought into contact with theoxygen gas 37. Oxygen atoms then diffuse from the oxygen gas 37 into thegallium oxide film. The oxygen atoms that have diffused into the galliumoxide film enter into the oxygen vacancies in the gallium oxide film.The oxygen vacancies are filled with the oxygen atoms and hence areeliminated. Since many oxygen vacancies are eliminated in the galliumoxide film, the oxygen vacancies in the gallium oxide film decrease at agreat degree. Particularly because the gallium oxide film has beenheated, oxygen atoms easily enter the oxygen vacancies. The oxygenvacancies in the gallium oxide film can accordingly be decreasedefficiently. Thus, the gallium oxide film having few oxygen vacanciescan be obtained.

As mentioned above, the film forming method in the first embodimentenables a gallium oxide film having few oxygen vacancies to be formed.The gallium oxide film is a semiconductor, and the oxygen vacancies inthe gallium oxide film function as donors of an n-type. This filmforming method can suppress manufacturing variation in a carrier densityof the gallium oxide film by decreasing a density of the oxygenvacancies in the gallium oxide film. Characteristics of the galliumoxide film can thus be controlled accurately.

In the film forming method of the first embodiment, the film formingprocess and the oxygen annealing process are performed in a same furnace20. The gallium oxide film is thus not brought into contact with theatmosphere external to the furnace 20 during transition from the filmforming process to the oxygen annealing process. This can preventunintended atoms from entering into the oxygen vacancies.

Second Embodiment

A film forming apparatus 100 in a second embodiment shown in FIG. 2 isan apparatus configured to epitaxially grow a gallium oxide film on asurface of a substrate 112. The film forming apparatus 100 is used formanufacturing a semiconductor device including the gallium oxide film.The film forming apparatus 100 comprises a first mist supply device 191,a second mist supply device 192, and a furnace 120.

The furnace 120 is a tubular furnace extending from an upstream end 120a to a downstream end 120 b. A downstream end of a solution mist supplypath 130 is connected to the upstream end 120 a of the furnace 120. Thedownstream end 120 b of the furnace 120 is connected to an ejection pipe128. The furnace 120 includes a substrate stage 126 therein configuredto support the substrate 112. The substrate stage 126 is configured tohave the substrate 112 be inclined relative to a longitudinal directionof the furnace 120. A heater 127 is disposed along an outer peripheralwall of the furnace 120. The heater 127 is configured to heat the outerperipheral wall of the furnace 120, which in turn heats the substrate112 in the furnace 120.

The first mist supply device 191 comprises a reservoir 140, a water tank154, and an ultrasonic transducer 148. The reservoir 140 is an enclosedcontainer. The reservoir 140 stores a solution 142 containing water(H₂O) and a raw material of the gallium oxide film dissolved in thewater. There is a space 144 between a surface 142 a of the solution 142and an upper surface of the reservoir 140. A film constitutes a bottomsurface of the reservoir 140. The water tank 154 is a container with itstop opened, and stores water 158 therein. The reservoir 140 has itsbottom immersed in the water 158 in the water tank 154. The ultrasonictransducer 148 is arranged at a bottom surface of the water tank 154,and is configured to apply ultrasonic vibration to the water 158 in thewater tank 154. When the ultrasonic transducer 148 applies ultrasonicvibration to the water 158 in the water tank 154, the ultrasonicvibration is transferred to the solution 142 via the water 158. Thesurface 142 a of the solution 142 then vibrates, thus mist of thesolution 142 (hereinafter termed solution mist 146) is generated intothe space 144 above the solution 142.

The upper surface of the reservoir 140 is connected to an upstream endof the solution mist supply path 130. An outer peripheral wall of thereservoir 140 is connected to a downstream end of a carrier gas supplypath 150. An upstream end of the carrier gas supply path 150 isconnected to a carrier gas supply source (not shown). A first carriergas 152 is supplied from the carrier gas supply source to the carriergas supply path 150. The first carrier gas 152 is introduced into thespace 144 in the reservoir 140 via the carrier gas supply path 150. Thefirst carrier gas 152 is an inert gas. The first carrier gas 152 has anoxygen concentration equal to or less than 21 vol %. More specifically,the first carrier gas 152 contains no oxygen. The first carrier gas 152that has been introduced from the carrier gas supply path 150 into thespace 144 flows from the space 144 to the solution mist supply path 130.At this occasion, the solution mist 146 in the space 144 flows to thesolution mist supply path 130 together with the first carrier gas 152.

The downstream end of the solution mist supply path 130 is connected tothe upstream end 120 a of the furnace 120. A downstream end of a carriergas supply path 134 is connected to a point in the solution mist supplypath 130. An upstream end of the carrier gas supply path 134 isconnected to a carrier gas supply source (not shown). A second carriergas 135 is supplied from the carrier gas supply source to the carriergas supply path 134. The second carrier gas 135 that has flowed into thecarrier gas supply path 134 flows into the solution mist supply path130. The solution mist 146 is therefore diluted in the solution mistsupply path 130. The second carrier gas 135 is an inert gas. The secondcarrier gas 135 has an oxygen concentration equal to or less than 21 vol%. More specifically, the second carrier gas 35 contains no oxygen. Thesolution mist 146 flows in the solution mist supply path 130 to itsdownstream end together with the second carrier gas 135 and the firstcarrier gas 152, and flows into the furnace 120.

The second mist supply device 192 comprises a reservoir 160, a watertank 174, and an ultrasonic transducer 168. The reservoir 160 is anenclosed container. The reservoir 160 stores water (more specifically,pure water (H₂O)) 162. There is a space 164 between a surface 162 a ofthe water 162 and an upper surface of the reservoir 160. A filmconstitutes a bottom surface of the reservoir 160. The water tank 174 isa container with its top opened, and stores water 178 therein. Thereservoir 160 has its bottom sunk in the water 178 in the water tank174. The ultrasonic transducer 168 is arranged at a bottom surface ofthe water tank 174, and is configured to apply ultrasonic vibration tothe water 178 in the water tank 174. When the ultrasonic transducer 168applies ultrasonic vibration to the water 178 in the water tank 174, theultrasonic vibration is transferred to the water 162 via the water 178.The surface 162 a of the water 162 then vibrates, thus mist of the water162 (hereinafter termed water mist 166) is generated into the space 164above the water 162.

An upstream end of a water mist supply path 180 is connected to theupper surface of the reservoir 160. A downstream end of the water mistsupply path 180 is connected to the upstream end 120 a of the furnace120 via the solution mist supply path 130. A downstream end of an oxygengas supply path 170 is connected to an outer peripheral wall of thereservoir 160. An upstream end of the oxygen gas supply path 170 isconnected to an oxygen gas supply source (not shown). Oxygen gas 172 issupplied from the oxygen gas supply source to the oxygen gas supply path170. The oxygen gas 172 is introduced into the space 164 in thereservoir 160 via the oxygen gas supply path 170. The oxygen gas 172that has been introduced from the oxygen gas supply path 170 into thespace 164 flows from the space 164 to the water mist supply path 180. Atthis occasion, the water mist 166 in the space 164 flows to the watermist supply path 180 together with the oxygen gas 172. The water mist166 that has flowed to the water mist supply path 180 flows into thefurnace 120 together with oxygen gas 172.

Next, a film forming method using the film forming apparatus 100 will bedescribed. Here, a substrate constituted of a single crystal ofβ-gallium oxide is used as the substrate 112. A β-gallium oxidesemiconductor film is epitaxially grown on the substrate 112. An aqueoussolution containing water and a gallium compound (e.g., galliumchloride) dissolved in the water is used as the solution 142. Further,ammonium fluoride (NH₄F) which is a dopant material has been dissolvedin the solution 142 to add fluorine as dopants to the gallium oxidefilm. Nitrogen is used as the first carrier gas 152 and the secondcarrier gas 135.

Firstly, the substrate 112 is set on the substrate stage 126. After thesubstrate 112 has been set, a film forming process and a water mistannealing process are performed.

Firstly, the film forming process is performed. In the film formingprocess, firstly, the substrate 112 is heated with the heater 127. Here,a temperature of the substrate 112 is controlled to be 750° C. orapproximately 750° C. When the temperature of the substrate 112 hasstabilized, the ultrasonic transducer 148 is activated to generate thesolution mist 146 into the space 144 in the reservoir 140. The firstcarrier gas 152 is introduced from the carrier gas supply path 150 intothe reservoir 140. Here, the first carrier gas 152 is controlled to flowat a flow rate of 5 L/min. The solution mist 146 then flows into thesolution mist supply path 130 together with the first carrier gas 152.Further, the second carrier gas 135 is introduced from the carrier gassupply path 134 into the solution mist supply path 130. Here, the secondcarrier gas 135 is controlled to flow at a flow rate of 5 L/min. As aresult of this, the solution mist 146 is diluted in the solution mistsupply path 130. The solution mist 146 flows into the furnace 120together with the first carrier gas 152 and the second carrier gas 135.The solution mist 146 flows through the furnace 120 from the upstreamend 120 a to the downstream end 120 b together with the first carriergas 152 and the second carrier gas 135, and is ejected to the ejectionpipe 128. The substrate 112 supported by the substrate stage 126 issupported in an orientation that allows the solution mist 146 flowingthrough the furnace 120 from the upstream end 120 a toward thedownstream end 120 b to be applied to the surface of the substrate 112.In doing this, apart of the solution mist 146 adheres to the surface ofthe substrate 112. Since the substrate 112 has been heated with theheater 127, a chemical reaction of the solution mist 146 (i.e., thesolution 142) takes place on the substrate 112. As a result of this,β-gallium oxide is generated on the substrate 112. The solution mist 146is continuously supplied to the surface of the substrate 112, and thus asingle-crystal β-gallium oxide film (semiconductor film) is epitaxiallygrown on the surface of the substrate 112. This condition enables thegallium oxide film to be grown at a rate equal to or more than 1 μm/hour(more specifically, a rate of 1.8 μm/hour or approximately 1.8 μm/hour).The gases that flow into the furnace 120 (i.e., the first carrier gas152 and the second carrier gas 135) have a low oxygen content, and thusthe solution mist 146 is more difficult to oxidize. More specifically,the gallium oxide compound and the dopant material both contained in thesolution mist 146 are more difficult to oxidize. This suppresses dustfrom adhering to the surface of the growing gallium oxide film. Sincethe gas flowing into the furnace 120 (i.e., the first carrier gas 152and the second carrier gas 135) has a low oxygen content, fluorine whichis dopants easily enter the oxygen sites in the gallium oxide film. Thegallium oxide film of n-type is therefore grown. Since the gas flowinginto the furnace 120 (i.e., the first carrier gas 152 and the secondcarrier gas 135) has a low oxygen content, oxygen vacancies are formedin the gallium oxide film while the gallium oxide film is grown. Inparticular, by growing the gallium oxide film at a high film formingspeed (i.e., a speed equal to or more than 1 μm/hour) as describedabove, many oxygen vacancies are formed. After the gallium oxide filmhas been formed, the ultrasonic transducer 148 is stopped to stop thesupply of the first carrier gas 152 and the second carrier gas 135.

Next, the water mist annealing process is performed. In the water mistannealing process, firstly the substrate 112 is heated by the heater127. Here, the temperature of the substrate 112 and the gallium oxidefilm is controlled to be 800° C. or approximately 800° C. In otherwords, in the water mist annealing process, the substrate 112 and thegallium oxide film are heated at a temperature higher than thetemperature of the substrate 112 in the film forming process. When thetemperature of the substrate 112 has stabilized, the ultrasonictransducer 168 is activated to generate the water mist 166 into thespace 164 in the reservoir 160. The oxygen gas 172 is introduced fromthe oxygen gas supply path 170 into the reservoir 160. The water mist166 then flows into the water mist supply path 180 together with theoxygen gas 172. The water mist 166 then flows from the water mist supplypath 180 into the furnace 120 together with the oxygen gas 172. Thewater mist 166 flows through the furnace 120 from the upstream end 120 ato the downstream end 120 b together with oxygen gas 172, and is ejectedto the ejection pipe 128. The gallium oxide film on the substrate 112supported by the substrate stage 126 is brought into contact with thewater mist 166 and the oxygen gas 172. Oxygen atoms then diffuse intothe gallium oxide film from water (H₂O) that constitutes the water mist166 and from the oxygen gas 172 (O₂). The oxygen atoms that havediffused into the gallium oxide film enter into the oxygen vacancies inthe gallium oxide film. The oxygen vacancies are filled with the oxygenatoms and hence are eliminated. Since many oxygen vacancies areeliminated in the gallium oxide film, the oxygen vacancies in thegallium oxide film decrease at a great degree. Particularly because thegallium oxide film has been heated, oxygen atoms easily enter the oxygenvacancies. The oxygen vacancies in the gallium oxide film canaccordingly be decreased efficiently. Thus, the gallium oxide filmhaving few oxygen vacancies can be obtained.

As described above, the film forming method of the second embodiment isconfigured to form a gallium oxide film having few oxygen vacancies.Characteristics of a gallium oxide film can thus be controlledaccurately.

In the film forming method in the second embodiment, the film formingprocess and the water mist annealing process are performed in a samefurnace 120. The gallium oxide film is thus not brought into contactwith the atmosphere external to the furnace 120 during transition fromthe film forming process to the water mist annealing process. This canprevent unintended atoms from entering into the oxygen vacancies.

In the film forming method in the second embodiment, fluorine is used asthe donors. Fluorine belongs to Group 17 and easily enters into oxygensites. As such, if one of Group 17 elements is used as donors, thegrowing of a gallium oxide film makes it easier for the donors to enterinto the oxygen sites. Group 15 elements also easily enter into theoxygen sites similarly to Group 17 elements.

Third Embodiment

FIG. 3 shows a film forming apparatus 200 in a third embodiment. Thefilm forming apparatus in the third embodiment is configured todischarge mist or gas from a nozzle 210 toward a plurality of substrates212. In the third embodiment, description of devices configured tosupply the mist or gas (e.g., a reservoir or a gas supply source) willbe omitted.

The film forming apparatus in the third embodiment includes a substratestage 226 configured to have the plurality of substrates 212 mountedthereon. A heater 227 configured to heat the substrates 212 is arrangedwithin the substrate stage 226. The plurality of substrates 212 isdisposed around a central axis 226 a of the substrate stage 226. Thesubstrate stage 226 rotates about the central axis 226 a. The nozzle 210is disposed above the substrate stage 226. The nozzle 210 and thesubstrate stage 226 are arranged in a furnace. The nozzle 210 has arectangular parallelepiped shape elongated in one direction. A pluralityof discharge ports 210 a aligned along a line is formed at a lowersurface of the nozzle 210. As shown by arrows 280, the mist or gasdischarged downwards from the discharge ports 210 a of the nozzle 210can be applied to an entirety of the substrate stage 226 in a diameterdirection of the stage 226. When the mist or gas is discharged from thenozzle 210 with the substrate stage 226 rotating, the mist or gas isapplied to all of the substrates 212 on the substrate stage 226. Whenthe substrate stage 226 is rotated at such a high speed that a movingspeed of a part of the stage where a moving speed of the substrates 212is highest becomes higher than a discharge speed of the mist or gas, alaminar flow of the gas is generated along surfaces of the substrates212. The mist or gas flows along the laminar flow, and then the entiretyof the surface of each substrate 212 is uniformly processed.Alternatively, the substrate stage 226 may be rotated at a low speed,and the laminar flow of gas may not be generated.

Next, a film forming method using the film forming apparatus 200 will bedescribed. Here, substrates constituted of a single crystal of β-galliumoxide are used as the substrates 212. A β-gallium oxide semiconductorfilm is epitaxially grown on each substrate 212. An aqueous solutioncontaining water and a gallium compound (e.g., gallium chloride)dissolved in the water is used as a solution for the solution mist.Further, β-carboxyethyl germanium sesquioxide ((GeCH₂CH₂COOH)₂O₃) whichis a dopant material has been dissolved in the solution to add germaniumas dopants to the gallium oxide film. Nitrogen is used as a carrier gas.

Firstly, the substrates 212 are placed on the substrate stage 226. Afterthe substrates 212 have been placed, a film forming process and a watervapor annealing process are performed.

Firstly, the film forming step is performed. In the film forming step,the substrates 212 are heated by the heater 227. Here, the temperatureof the substrates 212 is controlled to be 750° C. or approximately 750°C. When the temperature of the substrates 212 has stabilized, thesubstrate stage 226 is rotated, and the solution mist is discharged fromthe nozzle 210 together with the carrier gas. The solution mist thusadheres to the surface of each substrate 212, and a β-gallium oxide film(semiconductor film) is epitaxially grown on the surface of eachsubstrate 212. Since the carrier gas has a low oxygen content, thesolution mist is more difficult to oxidize. This suppresses the dustfrom adhering to the surface of the growing gallium oxide films. Sincethe carrier gas has a low oxygen content, germanium which is the dopantseasily enters oxygen sites in each gallium oxide film. The gallium oxidefilms of n-type are thus grown. In addition, since the carrier gas has alow oxygen content, many oxygen vacancies are formed in each galliumoxide film while the gallium oxide films are grown. After the galliumoxide films have been grown, the discharge of the solution mist and thecarrier gas is stopped.

Next, the water vapor annealing step is performed. In the water vaporannealing step, firstly, the substrates 212 are heated with the heater227. Here, the temperature of the substrates 212 and the gallium oxidefilms is controlled to be 800° C. or approximately 800° C. In otherwords, in the water vapor annealing step, the substrates 212 and thegallium oxide films are heated at a temperature higher than thetemperature of the substrates 212 in the film forming step. After thetemperature of the substrates 212 has stabilized, superheated watervapor (steam) is discharged together with the carrier gas from thenozzle 210. The surface of each substrate 212 is brought into contactwith the superheated water vapor. Oxygen atoms then diffuse from thewater vapor (H₂O) into each gallium oxide film. The oxygen atoms whichhave diffused into each gallium oxide film in turn enter the oxygenvacancies in the gallium oxide film. A partial pressure of the watervapor in the furnace is higher than a partial pressure of the watervapor in an atmosphere. Oxygen atoms therefore easily enter the oxygenvacancies. The oxygen vacancies are filled with oxygen atoms and therebydisappear. Since many oxygen vacancies are eliminated in each galliumoxide film, the oxygen vacancies in each gallium oxide film decrease ata great degree. Particularly because each gallium oxide film has beenheated, the oxygen atoms easily enter the oxygen vacancies. The oxygenvacancies in each gallium oxide film can accordingly be decreasedefficiently. Thus, the gallium oxide films each having few oxygenvacancies can be obtained. Alternatively, in the water vapor annealingstep, the water vapor may be heated with a heater not shown.

As described above, the film forming method in the third embodiment isconfigured to form gallium oxide film(s) each having few oxygenvacancies. Characteristics of a gallium oxide film can thus becontrolled accurately.

β-carboxyethyl germanium sesquioxide ((GeCH₂CH₂COOH)₂O₃) used in thethird embodiment as the dopant material is an organic matter, and hencemay remain in the furnace after the film forming process has takenplace. However, by supplying the high-temperature water vapor into thefurnace in the water vapor annealing process, the remaining organicmatter is oxidized quickly and is ejected as gas to the outside of thefurnace. This can prevent the organic matter from adhering to thegallium oxide film when the substrates 212 are being taken out of thefurnace after the water vapor annealing process has taken place. Assuch, the water vapor annealing process also provides an effect ofcleaning an inside of the furnace.

In another embodiment, a gallium oxide film of p-type may be epitaxiallygrown by using a solution which has a dopant material includingacceptors dissolved therein. Since oxygen vacancies function as itsdonors, by forming the gallium oxide film doped with the acceptors, thegallium oxide film can be prevented from becoming n-type.

Although the gallium oxide film is epitaxially grown in each of thefirst to third embodiments described above, a film constituted ofanother oxide may be epitaxially grown. The film to be epitaxially grownmay be a semiconductor, an insulator, or a conductor. Even if any oxidematerial is implemented, a film having a high crystallinity can beformed by filling its oxygen vacancies with oxygen.

The film forming apparatus in each of the first to third embodimentsmentioned above may have a controller added thereto, and the controllermay automatically perform each process.

A relation between features in the embodiments and features in theclaims will hereinafter be described. The first carrier gas and thesecond carrier gas in the first to third embodiments are examples of acarrier gas in the claims. The solution mist in the first to thirdembodiments is an example of mist of a solution in the claims. Each ofthe oxygen gas, the water mist, and the water vapor in the first tothird embodiments is an example of a fluid comprising oxygen atoms inthe claims. The fluid may contain oxygen atoms at a molar concentrationhigher than that of an atmosphere.

Some of the features characteristic to the technology disclosed hereinwill be listed below. It should be noted that the respective technicalelements are independent of one another, and are useful solely or incombinations.

In the method of forming the oxide film disclosed herein as an example,the substrate may be heated in the bringing of the oxide film intocontact with the fluid.

In such a configuration, the oxygen atoms easily enter the oxygenvacancies, and the oxygen vacancies are efficiently reduced.

In the method of forming the oxide film disclosed herein as an example,the substrate may be heated at a first temperature in the epitaxialgrowth of the oxide film, and the substrate may be heated at a secondtemperature higher than the first temperature in the bringing of theoxide film into contact with the fluid.

In such a configuration, the oxygen atoms easily enter the oxygenvacancies, and the oxygen vacancies are efficiently reduced.

In the method of forming the oxide film disclosed herein, the oxide filmmay be a semiconductor.

Oxygen vacancies function as donors in an oxide semiconductor. Thus, theoxide semiconductor easily becomes n-type if the oxide vacancies aregenerated. In such a configuration, an oxide film with few oxygenvacancies (i.e., oxide semiconductor film) can be formed, and thus thecharacteristics of the oxide film can be controlled accurately.

In the method of forming the oxide film disclosed herein, the solutionmay comprise a dopant material including atoms which function as dopantsin the oxide film. The oxide film containing the dopants may beepitaxially grown in the epitaxial growth of the oxide film.

In such a configuration, the oxide film doped with the dopant (oxidesemiconductor film) can be formed. Also, in such a configuration,oxidization of the dopant material that is easily oxidized can besuppressed, and thus the oxide film suitably doped with the dopant canbe formed.

In the method of forming the oxide film disclosed herein, the dopant maybe replaceable with oxygen atom sites in the oxide film.

In such a configuration, the dopant easily enters the oxygen atom sitesin the oxide film while the oxide film is grown. This enables the oxidefilm doped with a greater amount of dopant to be formed.

In the method of forming the oxide film disclosed herein, the dopant maybe in Group 17 or Group 15.

In the method of forming the oxide film disclosed herein, the dopantsmay be acceptors.

In such a configuration, the oxide film can be prevented from becomingn-type due to b vacancy of oxygen.

In the method of forming the oxide film disclosed herein, the fluid maycomprise at least one of oxygen gas, water vapor, and water mist.

In such a configuration, the oxygen atoms can be suitably supplied tothe oxide film.

In the method of forming the oxide film disclosed herein, the fluid maycomprise a processing gas constituted of oxygen gas or water vapor.Also, a partial pressure of the processing gas in the fluid may behigher than a partial pressure of the processing gas in an atmosphere.

In such a configuration, the oxygen atoms easily enter the oxygenvacancies, and the oxygen vacancies are efficiently reduced.

While specific examples of the present disclosure have been describedabove in detail, these examples are merely illustrative and place nolimitation on the scope of the patent claims. The technology describedin the patent claims also encompasses various changes and modificationsto the specific examples described above. The technical elementsexplained in the present description or drawings provide technicalutility either independently or through various combinations. Thepresent disclosure is not limited to the combinations described at thetime the claims are filed. Further, the purpose of the examplesillustrated by the present description or drawings is to satisfymultiple objectives simultaneously, and satisfying any one of thoseobjectives gives technical utility to the present disclosure.

What is claimed is:
 1. A method of forming a semiconductor gallium oxidefilm, the method comprising: supplying a mist of a solution comprising amaterial of the gallium oxide film dissolved therein to a surface of asubstrate together with a carrier gas having an oxygen concentrationequal to or less than 21 vol % so as to epitaxially grow the galliumoxide film on the surface of the substrate; and contacting the galliumoxide film with a fluid comprising oxygen atoms after the epitaxialgrowth of the gallium oxide film, wherein the gallium oxide film and thesubstrate are heated while contacting the gallium oxide film with thefluid, and after contacting the gallium oxide film with the fluid, thegallium oxide film is a single-crystal.
 2. The method of claim 1,wherein the carrier gas does not comprise oxygen.
 3. The method of claim2, wherein the substrate is heated at a first temperature in theepitaxial growth of the gallium oxide film, and the substrate is heatedat a second temperature higher than the first temperature whilecontacting the gallium oxide film with the fluid.
 4. The method of claim1, wherein a crystallinity of the gallium oxide film is improved bycontacting the gallium oxide film with the fluid.
 5. The method of claim4, wherein the solution comprises a dopant material comprising atomswhich function as dopants in the gallium oxide film, and the galliumoxide film containing the dopants is epitaxially grown in the epitaxialgrowth of the gallium oxide film.
 6. The method of claim 5, wherein thedopants are replaceable with oxygen sites in the gallium oxide film. 7.The method of claim 5, wherein the dopants are in Group 17 or Group 15.8. The method of claim 5, wherein the dopants are acceptors.
 9. Themethod of claim 1, wherein the fluid comprises at least one of oxygengas, water vapor, and water mist.
 10. The method of claim 1, wherein thefluid comprises a processing gas comprising oxygen gas or water vapor,and a partial pressure of the processing gas in the fluid is higher thana partial pressure of the processing gas in an atmosphere.
 11. A methodof manufacturing a semiconductor device comprising a semiconductorgallium oxide film, the method comprising: supplying a mist of asolution comprising a material of the gallium oxide film dissolvedtherein to a surface of a substrate together with a carrier gas havingan oxygen concentration equal to or less than 21 vol % epitaxially growthe gallium oxide film on the surface of the substrate; and contactingthe gallium oxide film with a fluid comprising oxygen atoms after theepitaxial growth of the gallium oxide film, wherein the gallium oxidefilm and the substrate are heated while contacting the gallium oxidefilm with the fluid, and after contacting the gallium oxide film withthe fluid, the gallium oxide film is a single-crystal.
 12. A filmforming apparatus configured to form a semiconductor gallium oxide film,the apparatus comprising: a mist supply device configured to supply amist of a solution comprising a material of the gallium oxide filmdissolved therein together with a carrier gas having an oxygenconcentration equal to or less than 21 vol %; a fluid supply deviceconfigured to supply fluid comprising oxygen atoms; and a controllerconfigured to control the mist supply device and the fluid supplydevice, wherein the controller is configured to: supply the mist fromthe mist supply device to a surface of a substrate together with thecarrier gas to epitaxially grow the gallium oxide film on the surface ofthe substrate; and supply the fluid from the fluid supply device to thegallium oxide film to contact the gallium oxide film with the fluidafter the epitaxial growth of the gallium oxide film, wherein thegallium oxide film and the substrate are heated while contacting thegallium oxide film with the fluid, and after contacting the galliumoxide film with the fluid, the gallium oxide film is a single-crystal.13. The method of claim 11, wherein the carrier gas does not compriseoxygen.
 14. The method of claim 11, wherein a crystallinity of thegallium oxide film is improved by contacting the gallium oxide film withthe fluid.
 15. The film forming apparatus of claim 12, wherein thecarrier gas does not comprise oxygen.
 16. The film forming apparatus ofclaim 12, wherein a crystallinity of the gallium oxide film is improvedby contacting the gallium oxide film with the fluid.